On the first 3 exams of her language class, Ashley got an average score of 87. What does she need on the next exam to have an overall average of 90?
Explanation: Let her score on the next exam be $x$ The sum of all of her scores is then $3 \cdot 87 + x$ The same sum must also be equal to $4 \cdot 90$ Solve: $x = 4 \cdot 90 - 3 \cdot 87 = 99$.